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Numerical simulation of wood drying
A mathematical and then a numerical model were developed for simulating a convective batch lumber drying process. The model incorporates mass and heat transfer relationships within the lumber stack, as well as thermodynamic properties of the wood and drying air. It takes into account a change of air properties along the stack and its effect on the mass and heat transfer parameters. Kiln and individual board properties as well as a drying schedule are the input parameters that are defined and entered by a user. The model relies on the drying rate functions which are empirical correlations based on single-board tests. The drying rate function for hemlock was obtained based on experiment results from 23 small charges dried over a range of conditions used in industry. Three larger batches of hemlock were also dried using three different industrial schedules. The change of average moisture content with time predicted by the model was verified by weighing a kiln charge with load cells during drying. The change of board temperatures and temperature drop along the stack were verified by measuring the actual temperatures in the kiln during drying. The model was first validated against data available in the literature. Then the experimentally-determined drying function for hemlock was used as the model input and the model output was compared to the larger hemlock batches. Validated variables for both cases were board temperatures, temperature drop through the package and average moisture content of the package